Dc fluorescent lamp with improved efficiency

ABSTRACT

A DC fluorescent lamp unit in which resistance wire is wound about the tubular lamp envelope for providing a temperature gradient along the length of the lamp which is operative to shift higher the bounds of the mercury vapor pressure gradient within the lamp envelope. A protective covering of insulation is provided by encasing the wire wound lamp envelope in clear plastic. The resistance wire winding may be connected to one of the lamp electrodes to additionally serve as a series ballast for the lamp.

United States Patent 1191 Roche 1 1 Jan. 30, 1973 54 DC FLUORESCENT LAMP WITH 2,411,601 11/1946 Spencer ..315 94 x IMPROVED EFFICIENCY I E emmers.... 1 lnventorl William Rflche, Merrimac, MHSS- 3,336,502 8/1967 011mm ..3l3/I08 R x TE S l I ted [73] Aqglgnee C y vama corpora Primary ExaminerAlfred L. Brody Filed? y 3,1971 A!!0rney-Norman J. OMalley, Edward 1. Coleman 1211 Appl No.: 139,552 and Ryan [57] ABSTRACT [52] US. Cl ..3l3/l08 R,33l153//934733115716005 A DC fluorescent p unit in which resistance wire is wound about the tubular lam envelo e for rovidin [51] 1111 L1 "01 1/62 P P P g a temperature gradem along the lfingth of the p [581 Field of Search ..3l3/l08 R,37,44,315/94, huh I h h b d f h Sum 0560 61 w 1c 1s operat1vetos1t g er e oun s 0 t e mercury vapor pressure grad1ent within the lamp envelope. A protective covering of insulation is provided [56] References cued by cncasing the wire wound lamp envelope in clear UNTED STATES PATENTS plastic. The resistance wire winding be connected I to one of the lamp electrodes to addmonally serve as :1 3,382,406 5/l968 Sehiavone ..3l5/60 X series ballast for the 2,030,437 2/1936 Francis etal. 1,984,428 12/1934 Pirani j ..3l5/61 X 8 Claims, 3 Drawing Figures a 2| IO PATENTEbJAna'o ma 6.714.492

n L N +AN (DENSITY 0F SATURATED VAPOR AT MERCURY HIGHER ATOM TEMPERATURE DENSITY DENSITY-b =N OF SATURATED VAPOR AT 40C WILLIAM J. ROCHE INVENTOR FIG. 3 BY ATTORNEY DC FLUORESCENT LAMP WITH IMPROVED EFFICIENCY BACKGROUND OF THE INVENTION This invention relates generally to fluorescent lamps and particularly to those operated from a direct current power supply.

The requirement for direct current (DC) operation of fluorescent lamps arises in a number of applications in which the power supply is limited to DC, for example on subway trains. A chronic disadvantage in the DC operation of fluorescent lamps, however, has been the significant efficiency degradation occasioned by mercury migration, a phenomenon which appears unavoidable in the DC operation of mercury vapor discharge devices. Mercury migration, often referred to as mercury pumping, is a relatively familiar phenomenon in the fluorescent lamp field and refers to the tendency of the mercury ions in a unidirectional mercury vapor discharge to migrate toward the cathode end of the lamp. As a result, the anode end of the lamp is starved of mercury, whereupon light output and efficiency are substantially reduced.

The typical solution to this problem has been to periodically reverse the direction of current flow in the lamp by means of a power supply switching mechanism to prevent excessive migration of mercury ions in one direction and thereby improve the efficiency and light output of DC operated fluorescent lamps. The optimum period between current reversals depends upon the lamp characteristics and power loading, for example, it can range from a reversal every half hour to a reversal every 12 hours. The means for accomplishing current polarity reversal may comprise an automatic timer switch on the terminals of the DC power supply, and in the case of subway trains has been provided by trip mechanisms at predetermined points on the train route.

Although providing some degree of improvement, the polarity switching technique is not convenient or economically feasible for many applications. Further, periodic current reversals do not provide an intrinsic improvement in lamp operation.

SUMMARY OF THE INVENTION Accordingly, it is an object of the prevent invention to provide an improved fluorescent lamp unit for DC operation.

It is another object of the invention to provide a substantial improvement in DC fluorescent lamp operation without the need for polarity reversing of the DC power supply.

A further object is to provide a fluorescent lamp unit having means for intrinsically improving its operation eliminating the migration of mercury ions, this shifting of the pressure gradient provides a much more useful vapor pressure range throughout the lamp.

In the preferred embodiment, the temperature gradient is provided by a resistance wire wound about the tubular lamp envelope. The resistance wire is connected to one of the lamp electrodes to additionally provide a series ballasting function. A protective insulating cover is provided by encasing the wire wound lamp envelope with a clear plastic material.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:

FIG. 1 isa generalized graph showing the density of mercury atoms as a function of lengthwise position in a fluorescent lamp tube under DC operating conditions, the abscissa being an illustrative outline of the lamp;

FIG. 2 shows one embodiment ofa lamp unit according to this invention; and

FIG. 3 is an enlarged detail section of the cathode end of the lamp unit of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT As noted in the summary, the present invention counteracts the deleterious effects of mercury migration in a DC fluorescent lamp by providing a thermal gradient along the length of the lamp, with the higher temperature toward the cathode. To provide a better understanding of the phenomenon of mercury migration and the effect ofa temperature gradient in providing improved performance, an outline of the theoretical considerations will now be presented.

DC lamps are in effect operated on a pulsating unidirectional current, with the same end always the cathode. In the mercury-rare-gas discharge, the positive ions are mercury ions, since mercury has a lower ionization potential than any of the rare gases. In a discharge tube in which the potential never reverses, the mercury ions are continually subject to a unidirectional force urging them to the cathode or negative end of the discharge tube. This force causes a transport of mercury ions at a rate F= n u E ions/cm -sec (l) where n is the average density of mercury ions, p. is the mobility of mercury ions (the ratio of drift velocity to electric field), and E is the average unidirectional voltage gradient (electric field) in the tube, in volts per unit of distance.

In the plasma of such a discharge tube, the electron density n is equal to the ion density 11 The electrons, having much higher mobility than ions, carry essentially all of the discharge current. The average current density can therefore be expressed using Ohms Law as,

J= NA GE (2) where J is the average current density of the plasma, a is the average conductivity of the plasma, I is the lamp current, A is the tube cross-section area, and E is the aforementioned electric field. For a fluorescent lamp 0- can be expressed as,

p. n eE (3) where p. is the electron mobility, n is the average electron density and e is the electronic charge. Substituting the value of a from equation (3) to equation (2) and solving equation (2) for E yields,

E =I/Af1 fl. Substituting the value of E from equation (4) into equation (l), and recalling that n, n yields +u+ )/(n u =(u+ (5) Equation (5) represents the flux of positive mercury ions being pumped toward the cathode end. This flux must be counter balanced by a return flux of neutral mercury atoms, diffusing back toward the anode under the influence of a gradient in the density of neutral mercury atoms in the gas (i.e., a gradient in the density of mercury vapor, or in the mercury vapor pressure).

The diffusion flux is (6) where D is the diffusion coefficient for mercury atoms in the gas, and dn/dx is the gradient in mercury atom density. In the steady state, as much mercury must be diffusing back (as neutral atoms) toward the anode as is being pumped toward the cathode as ions, or

dn/dx= (}L I/e,LL D/l) 7 it will be noted that the density gradient is independent of the density itself, and is proportional to average current and various constants p. ,p. ,D, characteristic of ions, electrons, and the rare gas. Solving equation (7) for n and substituting appropriate boundary conditions yields,

11 (NJ/M (x (8) where L is the distance between the cathode and anode electrodes in the lamp, and n1, is the value of n at x=L FIG. 1 represents aplot of equation (8) for values of m, equal to N and N+AN.

At no point in the discharge tube can the density of mercury atoms in the gas exceed that of a saturated vapor at the nearby wall temperature, since any excess will merely condense out. This maximum value corresponds to m, of equation (8). Thus, in a lamp in which the wall temperature is everywhere 40C, the density of mercury atoms as a function of position in the fluorescent lamp discharge tube will have the value indicated by the dashed line curve in FIG. 1', the value is equal to that of saturated vapor at the cathode (n, =N) and falls below that value elsewhere in the tube.

In the lamp in which the cathode end is externally heated to be at a higher temperature than the anode end, as taught by the present invention, the density of mercury atoms in the'tube will be as indicated by the solid line-curve in FIG. 1. Hence, from the curve of FIG. 1 it will be noted that, although the gradient of mercury pressure is unaffected, the introduction of a temperature gradient along the tube, with the cathode end hotter, does prevent total mercury starvation at the anode end by raising the value of n,, by an amount equal to AN.

For example, if the current is such that there is a 6 micron difference in mercury pressure between anode and cathode, and the cathode end wall temperature is 40C, such that the cathode end mercury pressure is 6 microns, the anode end mercury pressure will be zero. On the other hand, if the cathode end temperature is increased to 45C, such that the cathode end mercury pressure is 9 microns, and the anode end temperature is 40C or higher, the anode end mercury pressure will be 3 microns, and the tube will remain lighted along the entire length. The tube will have an optimum mercury pressure of6 microns at only one point along its length; however, as the curve of efficiency vs. vapor pressure is quite flat between 3 and 12 microns, and falls steeply below 3 microns, the overall efficiency of the lamp will be increased. By the use of a thermal gradient in accordance with the invention, therefore, the growing dark regions indicative of anode starvation are eliminated, thereby providing improved light output, and the overall efficiency of lamp operation is increased.

Referring now to FIGS. 2 and 3, a DC fluorescent lamp unit 1 is shown in accordance with one embodiment of the invention. The lamp has a sealed, hollow glass tube 2 of sufficient length to exhibit the dashed line curve of FIG. 1 and containing a suitable rare gas filling, such as for example percent argon. In addition, a charge of mercury is introduced into the tube, prior to sealing, to yield the necessary mercury vapor for the operation of the lamp. On the inside surface of the glass tube there is a coating 3 of phosphor which may be, for example, any suitable fluorescent lamp phosphor.

At one end of the glass tube 2 there is a conventional probe-type anode electrode 4 having a support and lead-in wire 5 sealed in a stem press 18 and internally connected to pin 16 ofa conventional single pin base 6. Disposed at the other end of the tube is a conventional filament-type cathode electrode 7, such as an oxidecoated tungsten coil. in this instance, support wires 8 and 9 of the cathode electrode are joined together and electrically connected to a resistance wire 10 at junction 11; for example, junction 11 may comprise a mechanical crimp joint. Wires 8 and 9 are shown sealed in a stem press 19 having an exhaust tube 20.

Wire 10 is run straight down the outside of tube 2 toward the anode end; then, commencing at a predetermined distance from the cathode end of the lamp, the wire 10 is wound about the glass tube, proceeding at a fixed winding rate toward the cathode end of the lamp. At the termination of the winding, wire 10 is internally connected to pin 15 of a conventional single pin base 12. Extending from'near joint 1 l to the starting point of the winding, wire 10 is secured to the glass tube 2 by a strip of transparent dielectric tape 13 to provide electrical insulation from the metallic portions of base 12 and the turns of the resistance wire about the lamp. Another strip of dielectric tape 14 insulates the terminal end of the wire 10 winding as it enters between tube 2 and metallic portions of base 12 for internal connection to the pin 15. Bases 6 and 12 are cemented to the ends of the lamp tube in the conventional manner,

the basing cement being denoted in Fit]. 3 by the numeral 21. Finally,'the wire wound glass tube is covered by a clear plastic insulating material 17. For example, a

sheet of Mylar polyester film (polyethylene terephthalate resin, Mylar is a trademark of E. l. du Pont de Nemours and Co.) about 4 mils in thickness may be wrapped about the glass tube and secured by tape at each end, the lamp then being subjected to a hot air blower to heat shrink the film on the glass tube.

With resistance wire wound and connected as illustrated, ignition of the lamp energizes the wire for providing an external source of heat. In particular, the winding provides localized heating of the lamp toward the cathode end. This produces the desired temperature gradient along the length of the tube, which in turn shifts the bounds of the mercury pressure gradient in the lamp to a higher level, thereby avoiding anode starvation and significantly enhancing lamp efficiency. In addition, resistance wire 10 is operative to provide a series ballast for the lamp between the cathode electrode 7 and pin 15.

For example, consider a 36 inch long 1.5 inch diameter fluorescent tube of standard construction. A suitable thermal gradient and ballast may be provided by spiraling about this lamp a resistance wire of 7 mils diameter and having a resistance of 8.75 ohms per linear foot. A wire length of 14.3 feet can be used to provide adesired ballast resistance of l25 ohms. The wire is wound about the lamp commencing at a point approximately three-fifths of the length of the tube from the cathode end and proceeding at a fixed winding rate of 1.7 turns per inch toward the cathode end of the lamp. Such an arrangement provides a thermal gradient of approximately 10C between the anode and cathode ends of the lamp.

It is to be understood however, that the method of providing a thermal gradient is not limited to the embodiment of FIGS. 2 and 3. A suitable temperature differential may also be provided in a more gradual manner by spiraling the entire length of the fluorescent tube with a wire of gradually increasing winding pitch toward the anode end. Further, the heater-ballast wire is equally suitable for use with lamps having nonlinear tubes or having all terminals at one end of the lamp. The wire need not serve as a ballast and may be separately energized. In lieu of a wire winding, the temperature gradient may be provided by other external sources of heat, such as by heat radiating strips on the tube or heat radiators within the lamp fixture.

What I claim is:

l. A fluorescent lamp unit comprising, a glass tube, a phosphor coating on the inside surface of said glass tube, an anode electrode disposed at one end of said tube, a cathode electrode disposed at the other end of said tube, mercury vapor contained within said tube, and means disposed outside of said glass tube for providing a temperature gradient along the length of said tube with the higher temperature toward the cathode end of said tube.

2. A lamp unit according to claim 1 wherein said means for providing a temperature gradient comprises a heat source externally disposed toward the cathode end of the glass tube.

3. A lamp unit according to claim 1 wherein said means for providing a temperature gradient comprises a wire externally wound about said glass tube and adapted to be electrically energized.

4. A lamp unit according to claim 3 wherein said wire is resistance wire and is connected to one of said electrodes to provide series bailasting for said lamp in addition to providing a temperature gradient.

5. A lamp unit according to claim 3 further including insulating means covering said wire winding on said glass tube.

6. A lamp unit according to claim 3 wherein said wire is wound about said glass tube commencing at a predetermined distance from the cathode end thereof and proceeding at a fixed winding rate toward the cathode end of said lamp.

7. A lamp unit according to claim 6 wherein said predetermined distance is approximately three-fifth of the length of said tube, and said winding rate is approximately 1.7 turns per inch.

8. A lamp unit according to claim 6 wherein said wire is a resistance wire connected to said cathode electrode for providing lamp ballasting, and further including a substantially clear plastic insulating material covering said wire wound glass tube. 

1. A fluorescent lamp unit comprising, a glass tube, a phosphor coating on the inside surface of said glass tube, an anode electrode disposed at one end of said tube, a cathode electrode disposed at the other end of said tube, mercury vapor contained within said tube, and means disposed outside of said glass tube for providing a temperature gradient along the length of said tube with the higher temperature toward the cathode end of said tube.
 2. A lamp unit according to claim 1 wherein said means for providing a temperature gradient comprises a heat source externally disposed toward the cathode end of the glass tube.
 3. A lamp unit according to claim 1 wherein said means for providing a temperature gradient comprises a wire externally wound about said glass tube and adapted to be electrically energized.
 4. A lamp unit according to claim 3 wherein said wire is resistance wire and is connected to one of said electrodes to provide series ballasting for said lamp in addition to providing a temperature gradient.
 5. A lamp unit according to claim 3 further including insulating means covering said wire winding on said glass tube.
 6. A lamp unit according to claim 3 wherein said wire is wound about said glass tube commencing at a predetermined distance from the cathode end thereof and proceeding at a fixed winding rate toward the cathode end of said lamp.
 7. A lamp unit according to claim 6 wherein said predetermined distance is approximately three-fifth of the length of said tube, and said winding rate is approximately 1.7 turns per inch. 